Do not create artifical name for extension-only filenames

[JanKrivanek]

Problem

Verification of files that starts with dot (e.g. .editorconfig) generates/uses snapshots which prepend nofilename to the tested file. This is a minor inconvenience - discussed here: #699 (comment)

Solution

filename is preserved as empty
directory structure is preserved as well
appropriate tests added

Note

Potentially similar approach might be taken for files without extensions (currently artifical .noextension extension appended). This would require some changes in EmptyFiles as well (to support empty extension). I'm leaving it for separate work if it ever becomes concern for anyone.

[SimonCropp]

so this is a breaking change?

[JanKrivanek]

so this is a breaking change?

That's correct.
More precisely:

• For updates from >= 18.3.1-beta.1 it's breaking
• For updates from <= 18.3.0 it's actualy enabling the upgrade without the breaking change that's currently imposed (as previously the 'dot filename' that was contained within some subdirectory was supported as is and in newer version the snapshot name is altered)

We are currently in the second category (trying to prevent sequence of 2 breaking changes)

[SimonCropp]

do u think it makes sense for this to handle deletion of the legacy file?

if no can u add a section in the description for how people can manually clean up

[JanKrivanek]

Good point.
Will look into that and update.

[JanKrivanek]

@SimonCropp I update the PR to rename the snapshots from previous naming convention (simple cleanup of previous naming convention is in previous commit - if that feels as better strategy). Let me know what are your thoughts on that.
Thanks!